1. Field of the Invention
The present invention relates to a method of detecting the oscillations which occur when an object, more particularly a glass plate, is damaged, in which method the oscillations are converted by sensors into electrical signals at frequencies lying within a definite frequency band, the sensor signals being processed in order to generate a warning signal, and to apparatus for carrying out this method with sensors for transforming the oscillations into electrical signals and a circuit arrangement to which the sensors are connected and which generates a warning signal in dependence on the oscillations which occur.
2. Description of the Prior Art
It is known, for example from German Pat. No. 2127562, that oscillations lying in the supersonic waverange occur when glass plates are broken or cut, while oscillations caused by environmental disturbances (for example when the glass plate undergoes shocks, knocks, etc.) comprise comparatively low frequencies of below about 100 kHz.
This is the basis of known devices for detecting damage to glass plates, which detect frequencies of oscillation above about 100 kHz and process them to yield corresponding warning or tripping signals. Such a device is known for example from German Pat. No. 2260352, in which piezo-electric transducers are cemented to the glass plate to be monitored. The output signals of these transducers are transmitted via screened cables or by radio to an evaluator circuit. This evaluator circuit evaluates the signals received from the sensors. Now in order to avoid false alarms caused by environmental effects, such as shocks, knocks, etc., this evaluation must take place in a complex fashion. For this reason, the evaluator circuit is of relatively complicated construction, which makes production and installation time-consuming and correspondingly expensive.
This disadvantage is eliminated in the case of the device described in German Pat. No. 2254540. The sensor takes the form of a freely oscillating piezo element, of which the characteristic frequency lies in the supersonic range to be monitored. The piezo element converts the oscillations which it receives into electrical signals, which are evaluated in a circuit accommodated in the same housing as the piezo element. According to the "goodness" of the piezo element, oscillations at frequencies lying in a more or less wide frequency band are now detected and evaluated. In the case of this last-named device, the circuit and appliance are considerably less technically complex than in the case of the device according to German Pat. No. 2260352, but as a result increased liability to false alarms must be expected since it is not possible with this freely oscillating piezo element to distinguish sufficiently clearly between oscillations generated by environmental effects and oscillations caused by cutting or breaking the glass plate.